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Flexible sandstone by Bill Baird The general public think of rocks as being hard, rigid and with a permanent unchanging form. Geologists know that the term rock can be extended to include less hard and rigid substances such as clays. However even geologists are surprised when they come across flexible rocks. Flexible sandstone occurs in several countries but the most famous is that from India. The peculiar rocks from Jind, near Delhi were originally formed from the decomposition of gneisses which contained a proportion of feldspar grains. On the subsequent decomposition of the feldspar grains the rock became a mass of loosely interlocking grains of quartz, with wide interspaces around them. Where the quartz grains of the sandstone interflnger with their neighbours, growth of the quartz crystals has taken place. This growth has created sites of articulation, rather like that of a human knee or elbow joint, thus allowing a surprising amount of flexibility in the roclc. Although the flexible sandstones from Jind do contain small quantities of the accessory minerals, kaolinite and mica, their unusual property is due solely to the overgrowth contacts created between the quartz grains during diagenesis. Bibliography Dussealt, M.B. 1980. Itacolumites: the flexible sandstones. Quarterly Journal of Engineering Geology 13, 119-128. Reed, F.R.C. 1949. The Geology of the British Empire. 2nd edition, p.416. Edward Arnold & Co., London. Wadia, D.N. 1966. Geology of India 3rd edition
(revised). MacMillan, New York.
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